1. Field of the Invention
The present invention relates to a lithographic projection apparatus, a device manufacturing method, and a method of manufacturing an optical element.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC""s). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as xe2x80x9calignment systemxe2x80x9d), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791.
In order to image ever smaller features, it has been proposed to use EUV radiation, with a wavelength in the range of from 5 to 20 nm, as the exposure radiation instead of UV, with a wavelength of 193 or 157 nm, as is used in current commercial lithography devices. No material capable of forming a refractive lens for optical elements in radiation and/or projections systems for EUV radiation is known so that the radiation and projection systems of an EUV lithographic apparatus must be made using mirrors, usually multilayer mirrors. The quality of the projected image is extremely sensitive to surface deformations (figure errors) in the mirrors, particularly those of the projection system. In order to prevent surface deformations caused by temperature variations, the mirrors are formed by depositing the multilayer stack on a substrate having the desired figure and made of a material having an extremely low, or zero, coefficient of thermal expansion (CTE). Various such materials are commercially available from a variety of suppliers. One, ZERODUR(trademark), is a glass ceramic made with various additives to provide the desired low CTE. While these materials have a very low CTE, the CTE is exactly zero at only one temperature so that some thermal expansion and contraction does take place, leading to surface deformations and a loss of image quality.
It is an aspect of the present invention to provide a lithographic projection system in which thermal effects of optical elements in the radiation and/or projection systems on imaging quality are further reduced or eliminated.
This and other aspects are achieved according to the invention in a lithographic apparatus including a radiation system constructed and arranged to provide a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate, wherein at least one component in the apparatus that in use experiences a heat load is made of a low coefficient of thermal expansion material having a coefficient of thermal expansion having a zero-crossing at a temperature between a manufacturing temperature and a mean operating temperature of that component.
By constructing the component using a material having a coefficient of thermal expansion zero-crossing temperature between the manufacturing and mean operating temperatures of the component, the thermal deformation of the component when operating is minimized or eliminated. Because the coefficient of thermal expansion will, in general, be negative below the zero-crossing temperature and positive above, as the temperature of the component changes from its manufacturing temperature to its operating temperature, the component will deform initially but those deformations will be reversed on the other side of the zero-crossing temperature.
The ideal temperature of the zero-crossing will depend on the shape of the coefficient of thermal expansion as a function of temperature in the region of the zero-crossing. If the coefficient of thermal expansion is linearly dependent on temperature in that region, a material should be used which has a coefficient of thermal expansion zero-crossing exactly midway between the manufacturing and mean operating temperatures. In general, the manufacturing, zero-crossing and mean operating temperatures should be such that the integral of the temperature dependent coefficient of thermal expansion between the manufacturing and mean operating temperatures is zero, or as close thereto as possible.
Where the material used is a glass, or a glass ceramic such as ZERODUR(trademark), the coefficient of thermal expansion zero-crossing temperature can be selected as desired by appropriate control of the additives and/or the manufacturing process. If necessary, a batch of glass (or glass ceramic) of the desired coefficient of thermal expansion zero-crossing temperature can be manufactured by trial and error. It will be appreciated that while the mean operating temperature of the apparatus might be determined by other considerations, the manufacturing temperature may be adjusted to enable use of a particular glass (or glass ceramic) having a coefficient of thermal expansion zero-crossing temperature that is fixed or only of limited variability.
The present invention may be applied to any component in a lithographic apparatus but is particularly advantageous when applied to optical elements in the path of the projection beam, especially mirrors in an EUV lithographic apparatus, especially those of the projection system, where surface deformations have the greatest effect on imaging quality, and those in the beginning of the illumination system, where the projection beam intensity is highest, leading to larger temperature variations. The present invention is also particularly applicable to small mirrors where the thermal load is concentrated, again leading to larger temperature variations. It will be appreciated that for multilayer mirrors, only the substrate, and not the multilayer stack, will be made out of the low coefficient of thermal expansion material.
It is generally preferably for consistency to make all of the optical elements affecting the projection beam in a lithography apparatus from the same batch of material so that if the operating temperatures of the different optical elements differ, the zero-crossing temperature may only be ideal for one element. In that case, the zero-crossing temperature is preferably chosen to be ideal for the mirror having the highest heat load, generally the first mirror.
According to a further aspect of the invention there is provided a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; using a patterning device to endow the projection beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material using a projection system, wherein at least one component in at least one of the radiation system and the projection system experiencing a heat load has a mean operating temperature and is made of a low coefficient of thermal expansion material such that the coefficient of thermal expansion zero-crossing temperature of the low coefficient of thermal expansion material is between the manufacturing temperature of the component and the mean operating temperature.
According to a still further aspect of the present invention there is provided a method of manufacturing an optical element that will, in use, experience a heat load and will be operated at a mean operating temperature, the method including selecting a low coefficient of thermal expansion material having a zero coefficient of thermal expansion at a first temperature; and manufacturing at least the optical element using the selected low-CTE material at a second temperature, wherein the first temperature is between the second temperature and the mean operating temperature so as to minimize surface deformation of the optical element, at the operating temperature.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of IC""s, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill in the art will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portion xe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.